Swiss Federal Institute for Forest, Snow and Landscape Research WSL

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Edition: Sat 21.12., 08:00
Avalanche Bulletin

Global warming accelerates CO₂ emissions from soil microbes

When Microorganisms decompose organic material in the soil, they actively release CO2 into the atmosphere. This process is called heterotrophic respiration. A novel model shows that these emissions could surge by up to 40 percent by the end of the century – most significantly in the polar regions.

Microbes release CO₂ when decomposing organic material in the soil. This emission has been increasing since the 1980s. (Photo: Markus Bolliger)

In brief

  1. It is projected that by the end of the century, CO2 emissions from soil microbes will escalate.
  2. Recent model estimates further show that microbial CO2 emissions in the polar regions will increase twice as much as in the rest of the world.
  3. In cold regions, the primary driver behind the higher CO2 emissions by microbes is attributed to soil moisture, whereas in other geographical zones, the rise in temperature is associated with increases in emissions.

The rise in atmospheric carbon dioxide (CO2) concentration is a primary catalyst for global warming, and an estimated one fifth of the atmospheric CO2 originates from soil sources. This is partially attributed to the activity of microorganisms, including bacteria, fungi, and other microorganisms that decompose organic matter in the soil utilizing oxygen, such as deceased plant materials. During this process, CO2 is released into the atmosphere. Scientists refer to it as heterotrophic soil respiration.

Based on a recent study published in the scientific journal Nature Communications, a team of researchers from ETH Zurich, the Swiss Federal Institute for Forest, Snow and Landscape Research WSL, the Swiss Federal Institute of Aquatic Science and Technology Eawag, and the University of Lausanne has reached a significant conclusion. Their study indicates that emissions of CO2 by soil microbes into the Earth's atmosphere are not only expected to increase but also accelerate on a global scale by the end of this century.

Using a projection, they find that by 2100, CO2 emissions from soil microbes will escalate, potentially reaching an increase of up to about forty percent globally, compared to the current levels, under the worst-case climate scenario. “Thus, the projected rise in microbial CO2 emissions will further contribute to the aggravation of global warming, emphasising the urgent need to get more accurate estimates of the heterotrophic respiration rates,” says Alon Nissan, the main author of the study, who is a Postdoc at ETH and researcher in the group ecological fluid dynamics of Markus Holzner at WSL.

Soil moisture and temperature as key factors

The scanning electron microscope image shows a colony of bacteria (coloured bluish) around a few micrometres small root (brown) in the soil. The image represents how bacteria and microbial communities influence CO₂ release from the soil through heterotrophic respiration: The soil contains soil grains to which microbial communities adhere and process dissolved oxygen O₂ and organic carbon DOC from organic matter (OM). (Graphic: Springer Nature / Institute of Environmental Engineering)

These findings do not only confirm earlier studies but also provide more precise insights into the mechanisms and magnitude of heterotrophic soil respiration across different climatic zones. In contrast to other models that rely on numerous parameters, the novel mathematical model, developed by Alon Nissan, simplifies the estimation process by utilising only two crucial environmental factors: soil moisture and soil temperature.

The model represents a significant advancement as it encompasses all biophysically relevant levels, ranging from the micro-scales of soil structure and soil water distribution to plant communities like forests, entire ecosystems, climatic zones, and even the global scale. Peter Molnar, a professor at the ETH Institute of Environmental Engineering, highlights the significance of this theoretical model which complements large Earth System models, stating, "The model allows for a more straightforward estimation of microbial respiration rates based on soil moisture and soil temperature. Moreover, it enhances our understanding of how heterotrophic respiration in diverse climate regions contributes to global warming."

Polar CO2 emissions likely to more than double

A key finding of the research collaboration led by Alon Nissan, Markus Holzner, Peter Molnar and their colleagues is that the increase in microbial CO2 emissions varies across climate zones. In cold polar regions, the foremost contributor to the increase is the decline in soil moisture rather than a significant rise in temperature, unlike in hot and temperate zones. Alon Nissan highlights the sensitivity of cold zones, stating, "Even a slight change in water content can lead to a substantial alteration in the respiration rate in the polar regions."

Based on their calculations, under the worst-case climate scenario, microbial CO2 emissions in polar regions are projected to rise by ten percent per decade by 2100, twice the rate anticipated for the rest of the world. This disparity can be attributed to the optimal conditions for heterotrophic respiration, which occur when soils are in a semi-saturated state, i.e. neither too dry nor too wet. These conditions prevail during soil thawing in polar regions.

On the other hand, soils in other climate zones, which are already relatively drier and prone to further desiccation, exhibit a comparatively smaller increase in microbial CO2 emissions. However, irrespective of the climate zone, the influence of temperature remains consistent: as soil temperature rises, so does the emission of microbial CO2.

How much CO2 emissions will increase by each climate zone

As of 2021, most CO2 emissions from soil microbes are primarily originating from the warm regions of the Earth. Specifically, 67 percent of these emissions come from the tropics, 23 percent from the subtropics, 10 percent from the temperate zones, and a mere 0.1 percent from the arctic or polar regions.

Significantly, the researchers anticipate substantial growth in microbial CO2 emissions across all these regions compared to the levels observed in 2021. By the year 2100, their projections indicate an increase of 119 percent in the polar regions, 38 percent in the tropics, 40 percent in the subtropics, and 48 percent in the temperate zones.

The graph on the left shows that heterotrophic soil respiration has been increasing since 1980, increasing twice as much in the polar zones (blue line) as the average increase for all climate zones (black line). Graph on the right: In the future, microbial CO₂ emissions from soils are likely to rise even more sharply, by 41.4 percent by 2100 according to a new model projection. In the polar regions, the increase will be significantly higher. (Graphics: Springer Nature / Institute of Environmental Engineering / Photo: Adobe Stock, Editing: Corporate Communications ETH Zurich)

Will soils be a CO2 sink or a CO2 source for the atmosphere?

The carbon balance in soils, determining whether soils act as a carbon source or sink, hinges on the interplay between two crucial processes: photosynthesis, whereby plants assimilate CO2, and respiration, which releases CO2. Therefore, studying microbial CO2 emissions is essential for comprehending whether soils will store or release CO2 in the future.

"Due to climate change, the magnitude of these carbon fluxes—both the inflow through photosynthesis and the outflow through respiration—remains uncertain. However, this magnitude will impact the current role of soils as carbon sinks," explains Alon Nissan.
In their ongoing study, the researchers have primarily focused on heterotrophic respiration. However, they have not yet investigated the CO2 emissions that plants release through autotrophic respiration. Further exploration of these factors will provide a more comprehensive understanding of the carbon dynamics within soil ecosystems.

  • The original press release from ETH Zurich can be found here.

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WSL and SLF provide the artwork for imaging of press articles relating to this media release for free. Transferring and saving the images in image databases and saving of images by third parties is not allowed.